1. Field of the Invention
The present invention relates to a silica-based single mode optical fiber used for optical transmission, and a method and apparatus for manufacturing the same.
2. Description of the Related Art
In recent years, vigorous studies has been made for increasing the transmission capacity in optical transmission using an optical fiber made of a silica-based glass.
In order to increase the transmission capacity in optical transmission, it is necessary for the optical fiber performing optical transmission to be capable of a single mode transmission under the wavelength used, because, if a plurality of modes are propagated within the optical fiber, a mode dispersion is unavoidably generated by a difference in the group velocity among propagation modes so as to deteriorate the signal waveform.
Therefore, a single mode optical fiber (SMF) having a zero dispersion wavelength around the wavelength of 1.3 μm has come to be used. Since the optical fiber of this type has a zero dispersion wavelength around the wavelength of 1.3 μm, it has been made possible to achieve optical transmission having the transmission distance exceeding 100 km and several hundreds of Mbps of transmission capacity around the wavelength of 1.3 μm.
On the other hand, it is desirable to carry out optical transmission using a wavelength around 1.55 μm because transmission loss of the optical fiber is rendered smallest around the wavelength noted above. Such being the situation, a dispersion shifted optical fiber (DSF) having a zero dispersion wavelength around the wavelength of 1.55 μm has been developed. The dispersion shifted optical fiber has made it possible to achieve optical transmission having a few Gbps of transmission capacity around the wavelength of 1.55 μm. Also, since this wavelength band is the gain band of an erbium-doped optical fiber amplifier, a drastic increase in the transmission distance has been brought about together with the increase in the transmission capacity.
Also, research and development on wavelength division multiplexing (WDM) optical transmission has been carried out vigorously in recent years as a technology for increasing transmission capacity. In this connection, vigorous studies are also being made of optical fiber that can be used suitably in WDM optical transmission.
Where an optical fiber is used for WDM optical transmission, it is required that the zero dispersion wavelength is not present in the wavelength band that is used in order to prevent four wave mixing. Also, in order to realize a WDM optical transmission system, it is generally required that waveform distortion of the transmitting optical signal that cannot be repaired in the relay point and the light receiving apparatus is not generated. To satisfy this requirement, it is said to be effective to suppress the nonlinear phenomenon caused by optical transmission line and to suppress the accumulated dispersion. Further, if there is a chromatic dispersion difference among the wavelengths of the optical signals, the waveform distortion amount is caused to be different for each wavelength. Therefore, it is necessary to reduce the dispersion slope in optical transmission line as much as possible.
A dispersion shifted optical fiber that does not have zero dispersion in the wavelength band used (NZDSF) has been developed as an optical fiber satisfying the requirements noted above. Four wave mixing scarcely takes place in NZDSF and the nonlinearity of NZDSF is sufficiently low. Therefore, NZDSF is being rapidly introduced and widely spreaded.
Also, a optical transmission line prepared by combining plural kinds of optical fibers so as to make the entire dispersion value and dispersion slope substantially zero has come to be used in many cases in WDM optical transmission systems. The known optical fibers used for this purpose include, for example, a dispersion compensation optical fiber (DCF) and a dispersion slope compensation optical fiber (DSCF).
Further, WDM optical transmission systems using a Raman amplification has also been studied in recent years, and study is also being made of utilizing the wavelength regions other than the wavelengths around 1.3 μm and 1.55 μm in WDM optical transmission.
A phenomenon of increasing transmission loss of optical fiber caused by the combination of a hydrogen molecule with a structural defect within the optical fiber is one of the phenomena obstructing optical transmission noted above. It is known in the art that the increase of transmission loss is caused by the absorption peak generated around the wavelength of 1.24 μm, around the wavelength of 1.38 μm, around the wavelength of 1.38 μm and on the longer wavelength sides thereof.
The particular phenomenon will now be described. In general, paramagnetic defects are present in an optical fiber. Among these paramagnetic defects, the non bridging oxygen hole center (NBOHC) and the per-oxy radical (POR) are said to affect the transmission characteristics, particularly, the long term stability of transmission loss.
The NBOHC noted above is a paramagnetic defect species that one of the four oxygen atoms combined with a Si atom has an unpaired electron that does not contributes to the combination with another atom, as shown in FIG. 1A. On the other hand, the POR noted above is a paramagnetic defect species that one of the four oxygen atoms combined with a Si atom is combined with another oxygen atom having an unpaired electron that does not contributes to the combination with another atom, as shown in FIG. 1B.
Particularly, if hydrogen is diffused in the optical fiber, the diffused hydrogen molecules are combined with these paramagnetic defects so as to generate an atomic combination generating the absorption peak within the transmission wavelength band. As a result, transmission loss is increased.
Particularly, in the case of using the Raman amplification system, the pumping light has a wave-length shorter than that of the amplified light by about 100 nm. For example, where a so-called “S-band” in the vicinity of 1,500 nm, which is outside of a gain wavelength band in the signal light amplifying system using an erbium-doped fiber (EDF), is amplified and utilized in the Raman system, the wavelength of the pumping light becomes 1,400 nm. This gives rise to the problem that, since the wavelength of 1,400 nm noted above is included in the wavelength region of so-called “OH absorption” falling within a range of between 1385 nm and 1410 nm, the pumping light is attenuated in the case where the OH absorption loss is large, resulting in failure to obtain a desired Raman gain.
Further, where the hydrogen molecules are diffused in the fiber, a problem is generated if a large amount of the NBOHC noted above is present in a region inside the mode field diameter (MFD region). Specifically, since the NBOHC reacts with the hydrogen molecule so as to form an OH radical, the OH absorption loss is increased with time so as to greatly impair the reliability of the system. The amount of such increase in the OH absorption loss is not mentioned in any international standard nowadays. However, the target value of the amount of such increase in the OH absorption loss is said to be 0.05 dB/km.
An example of an optical fiber in which the resistance to hydrogen is taken into account for suppressing the phenomenon of increasing transmission loss referred to above is disclosed in U.S. Pat. No. 6,131,415. It is disclosed that the hydrogen ion concentration is lowered in order to decrease transmission loss in the wavelength of 1385 nm, thereby making it possible to achieve optical transmission over the entire wavelength range of 1,200 nm to 1,600 nm.
Also, other examples of optical fibers in which the resistance to hydrogen is taken into account are disclosed in U.S. Pat. Nos. 5,838,866 and 6,128,928. It is disclosed that such an amount of germanium as not to substantially increase the refractive index is added to the inner cladding region positioned adjacent to the core so as to improve the resistance to hydrogen.
However, any of these U.S. patent specifications does not refer at all to, for example, the density of the paramagnetic defect such as the NBOHC or the POR within the optical fiber and, thus, the allowable limit of the density or the like of the paramagnetic defect in the optical fiber remains unclear.
Further, the prior art relating to the technology for decreasing the initial loss of the optical fiber or for maintaining the mechanical strength by improving the fiber drawing method is disclosed in, for example, Jpn. Pat. KOKAI Publication No. 2001-192228 and Jpn. Pat. Publication No. 2001-114526.